January 09, 2025

On the three-terminal regulator IC output voltage formula and usage - Power Circuit - Circuit Diagram

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=V××(1+R2/R1)+IdR2
Where V × × represents the output voltage of the IC. This is a complete formula and the result is accurate. The expression of V0 is related to the quiescent operating current Id of the fixed output regulator ground. When the Id changes, it will cause changes in the output voltage and load current. However, since the Id value is small, it is generally only a few milliamps, so the accuracy of the regulator is small. When the resistance values ​​of R1 and R2 are small, the IdR2 term can be ignored. At this point, the expression of V0 can be simplified to:
V0≈V××(1+R2/R1)
In order to make the results obtained in practice match the calculated values, the application conditions of the simplified expression should be selected: the values ​​of R1 and R2 must be selected to be small.
At present, some book articles often ignore the above conditions, and only recommend simplified expressions, which makes the contradiction between the practical results and the calculated values, often causing confusion to the reader.
It should be noted that the three-terminal fixed output voltage regulator integrated circuit 78×× (including 79××) series, externally added resistors to improve the application condition of the output voltage V0, according to the application data:
IdR2 can be ignored only when V××/R1>3Id, and V is allowed to be applied. Simplify the expression and match the practice with the calculated results.
Therefore should choose: R1 <V × × / 3Id (R2 should be based on R1)
Assume that 3Id=20mA, then the 7805 regulator should choose R1<0.25kΩ;
7812 regulator should choose R1 <0.6kΩ;
7815 regulator should choose R1<0.75kΩ;
The 7824 regulator should be R1 <1.2kΩ.
It can be seen that the simplified expression of V0 can be applied only if the value of R1 is met, otherwise the practice and calculation results may not match.



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